Method of production of porous glass base material for optical fiber with cleaning of the burner with gas at 25 m/s or faster

ABSTRACT

In a method of manufacturing a porous glass preform for an optical fiber which preform is formed as a deposit of fine glass particles by using a burner, a method to prevent the contamination of any glass particles having failed to be properly deposited so that the generation of voids may be minimized in a transparent glass preform which is produced by heating the porous glass preform. An inert gas is caused to flow through the burner at a rate of at least 25 m/s before fine glass particles are formed by the hydrolysis and/or oxidation of a glass material in a flame produced by the burner supplied with a mixture of a gas of the glass material and a gas for combustion and are deposited on a rotating starting member. It is desirable to elevate the pressure of the inert gas above the atmospheric pressure by a device connected to the burner by a pipeline, and cause it to flow rapidly through the burner.

TECHNICAL FIELD

This invention relates to a method of manufacturing a porous glasspreform for an optical fiber, and more particularly to an improvement inthe method of manufacturing a porous glass preform by depositing fineglass particles formed by hydrolysis in a flame produced by a burner.

BACKGROUND ART

One of the known methods of manufacturing a porous glass preform for anoptical fiber is the vapor phase axial deposition (VAD) method. In thismethod, as shown in FIG. 2, a mixture of a gas for combustion and a gasof a glass material is jetted out through a burner used for formingglass particles, hereafter simply ‘burner’ 2 (or 2′) to produce a flame3 (or 3′) in which the glass material is hydrolyzed or oxidized to formfine glass particles, and while the glass particles are deposited on thelower end of a rotating starting member 6, to form a deposited porousbody, the starting member 6 is moved relative to the burner 2 (or 2′)with the growth of the porous body, whereby a porous glass preform 1 isobtained. Although the method shown in FIG. 2 employs two burners, amethod using only one burner, or more than two is alternativelypossible. The porous glass preform 1 is heated in an electric furnace toform a transparent glass preform and it is drawn into an optical fiber.

In the conventional VAD method the burner 2 (or 2′) is provided with,for example, SiCl₄ as the glass material, and a fuel gas such as ahydrogen or hydrocarbon gas, and a gas assisting combustion, such asoxygen or air, as the gases for combustion. Fine glass particles (SiO₂)are formed by reaction of the following formula (I):

SiCl₄+2H₂O→SiO₂+4HCl  (I)

Not all of the glass particles that are formed, however, are depositedas the preform 1. Some of the glass particles float in the reactionvessel 4 and attach to its inner wall to form a layer of glass particlesthereon. If this layer grows to some extent in thickness, glassparticles are likely to fall off the reaction vessel wall, attach to thesurface of the porous glass preform 1 and form a gap therein. In thiscase, the voids may be formed when the preform is heated intotransparent glass.

As a method for solving this problem, the following method has beenproposed in Unexamined Published Japanese Patent Applications Nos.162642/1987 and 123831/1988. A heater and an outlet to a reaction vesselfor gas are added, and the gas heated to high temperature is forced toflow around the burner and the porous glass preform along the inner wallof the reaction vessel; thereby the gas is prevented from remaining nearthe inner wall of the reaction vessel; glass particles are preventedfrom attaching to the reaction vessel; the gas is caused to flow in thereaction vessel so as to retain the glass particles from floating in thereaction vessel. The burner for synthesizing glass particles also has aproblem with respect to the attachment and mixture of glass particles.Namely, when the mixture of the combustion gas and the glass material isjetted from the tip of the burner for synthesizing glass particles, apart of the gas mixture is likely to scatter around the burner andattach to the vicinity of its outlet as glass particles. The glassparticles are also likely to even enter the burner as a result of theirentrainment by the gas surrounding it. Moreover, even if the floating ofglass particles may be restrained during the synthesizing of glassparticles, by the method proposed in the above mentioned patentapplications, it is still likely that after the manufacturing of apreform is stopped, the glass particles may enter the burner during thecooling of the preform.

If the glass particles which have attached to the burner, or entered itas described above are left as they are, they are likely to leave theburner and attach to the surface of a preform during the subsequentpreform manufacturing. In this case also, the particles attach in amanner different from new particles produced in a flame and deposited onthe preform, and are likely to form voids when it is heated intotransparent glass. Moreover, the attached glass particles spoil theburner if they form transparent glass in the burner under the heat ofthe gas for combustion. Therefore, it is necessary to clean the burnerafter manufacturing of each preform by removing, by suction or othermeans, the glass particles which have attached or entered therein.

DISCLOSURE OF THE INVENTION

Although the methods proposed in the above mentioned patent applicationshave been somewhat effective for preventing the glass particles fromattaching to the inner wall of the reaction vessel of glass particles,the formation of voids in a preform by the glass particles attaching toit is still an outstanding problem.

The glass particles are likely to remain not only in the reactionvessel, but also in the burner even after it is cleaned as mentionedabove, and attach to a preform after starting of the synthesizing andbecome the cause of the voids. Namely, the attached glass particles mayfall of the reaction vessel wall or the outlet end of the burner whenmanufacturing of a preform is stopped, or may not be sucked from theburner completely even by very careful cleaning, and may enter it. Someof the falling glass particles sometimes enter deep into the burner, forexample, near its joint to a pipeline, and hence their removal bysuction is very difficult. The creating of a sufficiently large pressuredifference for removing any foreign matter from such a deep region inthe burner by suction is very likely to result in the destruction of itsglass wall having a thickness of, say, only 1 mm near its outlet end.The use of a new burner for manufacturing each preform is a very costlysolution to such a problem.

Under these circumstances, it is a subject of this invention to providea method which can prevent fine glass particles from attaching to aburner, or entering it, and thereby avoid the formation of voids in atransparent glass preform.

This subject is essentially attained by starting the deposition of fineparticles of glass after causing an inert gas to flow at a rate of atleast 25 m/s through a burner for producing those particles.

The inert gas preferably has a pressure elevated above the atmosphericpressure. The pressure of the inert gas is preferably elevated by apressurizer connected to the burner. The inert gas is preferably causedto flow at a rate of 25 to 50 m/s. When the inert gas is made to flow inthe burner for manufacturing glass particles the pressure is preferablyreduced by at least about 0.1 kPa in an exhaust pipe extending formreaction vessel in which the burner is mounted.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a schematic sectional view illustrating the method of thisinvention; and

FIG. 2 is a schematic sectional view illustrating the known method.

In the drawings, 1 is a porous glass preform, 2 and 2′ are each a burnerfor producing fine particles of glass, 3 and 3′ are each a flame, 4 is areaction vessel, 5 is an exhaust pipe, 6 is a starting material, 7 is apressurizer, 8 is a pipeline, and 9 is a valve.

BEST MODE FOR CARRYING OUT THE INVENTION

According to this invention, an inert gas is introduced into a burnerfor producing fine glass particles so as to flow at a rate of at least25 m/s therethrough before the deposition of fine particles of glassstarts to form a porous glass preform. Glass particles which areattached to the burner or have entered it and any other foreign matterscan easily be removed from the burner, and discharged from the mufflethrough an exhaust pipe by the inert gas flowing out of the burner. Evensuch particles or foreign matter entering the burner during or after itscleaning can be removed easily if the inert gas is introducedimmediately before the formation of a preform is started.

In this invention, the flow rate of the inert gas in the burner forproducing fine glass particles is 25 m/s or more, and particularlypreferably from 25 to 50 m/s. At a rate below 25 m/s, the gas may failto remove the entered glass particles thoroughly from the burner, whileat a rate above 50 m/s, it is likely to exert so heavy a load on thejoint between the burner 2 and a pipeline 8 (FIG. 1) or the burner 2itself to cause it to be broken. As a method for flowing the inert gasat a rate of at least 25 m/s, an inert gas having an elevated pressureis flowed, and its flow rate may be adjusted depending on its pressure.

The inert gas may be of any kind if it does not have any adverse effecton the burner, and if it does not contain any foreign matter, such as ametal powder, or dust, that may remain in a porous glass preform andeventually in an optical fiber and increase its transmission loss.Examples of the inert gas are nitrogen, argon and helium.

The invention will now be described more specifically with reference toFIG. 1. In this invention, the method for synthesizing the porous glasspreform itself is the same as the conventional method. A gas of theglass material and if required, a mixed gas containing a dopant gas, afuel gas, a gas for assisting combustion and an inert gas are suppliedto burners 2 and 2′ for synthesizing glass particles in a [muffle]reaction vessel 4. The glass material is hydrolyzed and/or oxidized inflames 3 and 3′ produced by the burners 2 and 2′ to form fine glassparticles, and those particles are deposited on a starting member 6, toform a porous glass preform 1. The gas in the reaction vessel 4 isexhausted through an exhaust pipe 5.

According to this invention, before starting the synthesizing of theporous glass preform 1, an inert gas of which pressure is elevated by apressurizer so as to flow at a rate of at least 25 m/s in the burner, isintroduced at a stretch into the burner through a pipe line and a valve,and thereby the inner part of the burner is cleaned. Although thepressurizer 7, pipeline 8 and valve 9 are shown only for the burner 2,the burner 2′ is also provided with a pressurizer, a pipeline and avalve, and is likewise supplied with an inert gas to clean it.

Here, “an inert gas is introduced at a stretch” mentioned above means amethod in which the valve 9 of the pipeline 8, the pressure of which iselevated, is opened in an instant and is kept open for about fiveseconds to allow the inert gas to flow and then the valve 9 is closed.More preferably, after the valve 9 is opened once, the valve 9 is shutand the pressure is elevated in the pipeline, and then the valve 9 isopened in an instant again and kept open for about five seconds to allowthe inert gas to flow; thus these processes are repeated several times.The number of times for which it is repeated depends on the degree ofcontamination of the inner part of the burner.

In this case, the pressure of the exhaust pipe 5 is preferably reducedby about 0.1 kPa or more. This is because the glass particles which haveflowed out of the burners 2, 2′ for synthesizing the glass particles areprevented from floating in the muffle, and according to the inert gasflow, the foreign matters are exhausted to the exhaust pipe from theneighborhood of the inner wall of the 4 reaction vessel are cleaned moreeffectively.

Although FIG. 1 shows the mode in which two burners 2 and 2′ are usedfor forming a porous glass preform 1, the method of this invention canalso be carried out by using only one burner, or more than two burners.

Although the foregoing description has been based on the VAD method,this invention is equally applicable to any other method of forming aporous glass preform from glass particles produced by a burner, such asthe outside vapor phasedeposition (OVD) method, to provide equallysatisfactory results in the cleaning of the burner.

EXAMPLE

As an example, the porous glass preform was manufactured using equipmenthaving the construction shown in FIG. 1 according to this invention.Each of the burners 2, 2′ had a diameter of 50 mm and a length of 500mm. Before starting the synthesizing, the pressure of nitrogen gas as aninert gas was elevated to 6 kg/cm² (or 588,399 Pa) by the pressurizer 7and then, the valve 9 was opened in an instant and was kept open forabout five seconds, and was shut. Then, the operations for elevating thepressure for about 20 seconds and opening the valve 9 for about 5seconds at a stretch were repeated three times. When the valves 9 wereopened, the inert gas flowed rapidly into the burners 2, 2′ through thepipelines 8. It flowed at a rate of 25 m/s through the burners 2, 2′.The pressure of the exhaust pipe 5 was reduced by 0.1 kPa.

The burner 2 was supplied with SiCl₄ at a rate of 0.2 liter per minute,GeCl₄ at a rate of 0.1 liter per minute, hydrogen at a rate of 20 litersper minute, oxygen at a rate of 30 liters per minute and argon at a rateof 10 liters per minute, while the burner 2′ was supplied with SiCl₄ ata rate of three liters per minute, hydrogen at a rate of 70 liters perminute, oxygen at a rate of 70 liters per minute and argon at a rate of20 liters per minute.

Thus, there were produced 10 porous glass preforms 1 each having adiameter of 150 mm and a length of 800 mm. Each preform was heated in anelectric furnace to form a transparent glass preform. The number ofvoids found in these transparent glass preforms was on the average only0.2 per piece of them.

For comparative purposes, 10 porous glass preforms each having the samesize as stated in the above example were manufactured using equipmenthaving the construction shown in FIG. 2, in the same manner with theexample except that no inert gas having an elevated pressure was flowedthrough the burners 2, 2′ for synthesizing glass particles beforestarting the formation of each preform. The porous glass preform thusobtained were heated under the same condition as in the above example soas to become transparent glass preforms, in which as many as 3.5 voidsper piece on the average were found.

From the result of the above-mentioned example and comparative example,it is confirmed that this invention can form a transparent glass preformhaving a very small number of voids as compared with the product of theconventional method.

As is obvious from the foregoing, this invention makes it possible toprevent the contamination of any porous glass preform by glass particlesattaching to, or remaining in the burners and thereby reduce greatly thevoids that may eventually be formed in any transparent glass preform foran optical fiber.

This invention requires only a simple apparatus and a simple operation,and can be carried out even immediately before the formation of a porousglass preform is started. The cleaning of the burners prolongs theirlife and enables them to be used repeatedly for making many preforms.Therefore, this invention can very effectively reduce the cost ofmanufacturing porous glass preforms for optical fibers.

What is claimed is:
 1. A method of manufacturing a porous glass preform for an optical fiber by depositing on the periphery of a rotating starting member, fine glass particles formed by the hydrolysis and/or oxidation of SiCl₄ in a flame, said flame is produced by at least one burner supplied with a mixed gas containing a gas of SiCl₄ and a gas for combustion, wherein an inert gas is caused to flow through said burner at a rate of at least 25 m/s before starting synthesis of said porous glass preform and after the formation of the previous glass preform.
 2. A method as defined in claim 1, wherein said inert gas has a pressure elevated above the atmospheric pressure.
 3. A method as defined in claim 2, wherein said pressurizer is elevated by a pressurized connected to said burner through a valve located in a different pipe line from the one for said mixed gas.
 4. A method as defined in claim 2, wherein said pressure is elevated by a pressurizer connected to said burner and said inert gas is introduced at a stretch.
 5. A method as defined in claim 1, wherein said rate is from 25 to 50 m/s.
 6. A method as defined in claim 1, wherein said burner is mounted in a reaction vessel having an exhaust pipe, and the pressure of said exhaust pipe is reduced by at least about 0.1 kPa.
 7. A method of manufacturing a porous glass preform for an optical fiber by depositing on the periphery of a rotating starting member fine glass particles formed by the hydrolysis and/or oxidation of SiCl₄ in a flame, said flame is produced by at least one burner supplied with a mixed gas containing a gas of said SiCl₄ and a gas for combustion wherein an inert gas is caused to flow through said burner at a rate of at least 25 m/s without said mixed gas before starting synthesis of said porous glass preform. 